1. Field of the Invention
This invention relates to the field of reforming hydrocarbons in the presence of a reforming catalyst. More specifically, this invention relates to heat exchange between a reforming feedstream and a reforming effluent stream in a reforming process.
2. Discussion of the Prior Art
Catalytic reforming is a well-known process used in petroleum refineries to increase the octane number of straight run distillates (naphthas) by promoting chemical reactions which reduce the paraffin content and increase the content of aromatics and isoparaffin fractions. The desired chemical reactions are carried out over a catalyst at temperatures in excess of 900.degree. F., and in the most common processes, at pressures less than 150 psi. In a typical reformer a hydrotreated naphtha with a 380.degree. F. end point is mixed with a recycle gas which is rich in hydrogen and heated by indirect heat exchange with gaseous products from the reactor in a feed-effluent exchanger. About 30-40% of the heat transferred goes to vaporize the liquid feed. After the combined feed leaves the feed-effluent exchangers it enters a preheater that raises the feed to the desired reactor temperature.
The major reactions that occur in the reformer are the dehydrogenation of naphthenes to form aromatics, the dehydrocyclization of paraffins to form aromatics, the isomerization of paraffins, and the hydrocracking of higher boiling fractions to form paraffins in butane. The dehydrogenation and dehydrocyclization reactions are endothermic and evolve hydrogen. These reactions are favored by reduced pressure, reduced space velocity and high temperature. The paraffin isomerization reaction is slightly endothermic and is not significantly influenced by pressure in the reforming zone. The hydrocracking reactions are undesirable exothermic reactions that are favored by increased pressure and low space velocity. Due to the nature of the above reactions, it is desirable to operate the process at as a low a pressure as possible. Lower pressures, however, increase the amount of coke deposited on the catalyst thereby reducing its effectiveness. Coking problems can be reduced by increasing the ratio of hydrogen to hydrocarbons in the feed or by using more coke-tolerant catalyst. Further information on reforming processes may be found in U.S. Pat. Nos. 4,119,526; 4,409,095 and 4,440,626.
Proper heat recovery in the feed effluent exchanger is important to the efficiency of product recovery in the reforming process. After the reaction occurs, the hot effluent exchanges heat with the feed by first cooling to the dewpoint and then partially condensing. The higher the feed outlet temperature to the preheater, the less fuel must be burned to maintain reactor temperature. Also, the more condensation that occurs in the effluent stream in the exchanger, the less downstream cooling must be provided to separate the reformate from the recycle gas.
It has been difficult by conventional methods to increase the outlet temperature of the reforming feedstream or the amount of condensation that occurs in the effluent stream. The typical approach of a person skilled in the design of feed effluent changers, when attempting to decrease the temperature difference between the feed leaving and the effluent entering the heat exchanger, is to increase the heat recovery or capacity of the existing system by adding surface area, either in the form of longer exchangers or additional units in parallel. Both approaches are costly and have technical disadvantages. Firstly, increasing the exchanger length would result in additional pressure drop that translates to increased recycle gas compressor power and reduced delivery pressure to downstream processes. Secondly, the addition of extra tubes, either as larger diameter exchangers or more units in parallel, would result in lower fluid velocities since the total stream is split into more paths. The resulting lower velocities would in turn lead to lower heat transfer coefficients which would work against the addition of surface area and also increase the likelihood of undesirable maldistribution of the 2-phase feedstream at the inlet. Finally, the designer would face the problem of reduced available temperature difference between the feed and effluent streams. Each increment in heat recovery gained by the addition of ordinary surface area results in a reduction of the temperature difference. Accordingly, the designer is confronted with an approaching temperature pinch and declining heat transfer coefficients with lower velocities and would likely conclude that exorbitant amounts of additional bare tube surface area must be added to gain a very modest increase in heat recovery and, therefore, is not practical or feasible.
It is known in the art that the surface of heat exchanger tubes can be treated to promote nucleate boiling. Such nucleate boiling surfaces can promote dramatic increases in the boiling film coefficients that are associated with heat transfer tubes in a boiling heat exchange zone. Such enhanced boiling surface for heat exchange tubes are discussed in U.S. Pat. Nos. 3,384,154; 3,821,018; 4,064,914; 4,060,125; 3,906,604; 4,216,826 and 3,454,081. Such surfaces have been known to provide benefits to a variety of processes. For example, a significant improvement in the refrigeration of an alkylation effluent by the use of an enhanced boiling surface is taught in U.S. Pat. No. 4,769,511. Such nucleate boiling surfaces have been known to increase boiling film coefficients by a factor of 10 or more.
It is an object of this invention to increase the heat transfer between a reforming feedstream and a reforming effluent stream.
It is a further object of this invention to reduce the temperature differential between an existing reforming feedstream that is exchanged with an entering reforming effluent stream.
A further object of this invention is to increase the feed processing capacity of a reforming process while maintaining the same feed outlet temperature and effluent inlet temperature and the same surface area in a feed effluent heat exchanger.